Set cont-type header before get_callback_info

The purpose of this PR is to allow more flexibility for setting the "Content-Type" header inside of controllers.

If the "Content-Type" header is set prior to get_callback_info, I can set the "Content-Type" header for a full controller inside the \__init__ method

    class Default(Controller):
    """
    HTML response route handling
    """
    def __init__(self, request, response):
        """
        Set Content-Type header to application/json
        """
        super(Default, self).__init__(request, response)
        self.response.headers['Content-Type'] = "text/html"